The concept of bias is the lack of internal validity or incorrect assessment of the association
between an exposure and an effect in the target population in which the statistic estimated
has an expectation that does not equal the true value. Biases can be classified by the
research stage in which they occur or by the direction of change in a estimate. The most
important biases are those produced in the definition and selection of the study population,
data collection, and the association between different determinants of an effect in the …